The present invention relates to the preparation of sulphur hexafluoride, and more especially, to an improved process and apparatus for the production of sulphur hexafluoride from elementary fluorine and sulphur, with the improvement comprising the elimination and/or recycle of the higher-boiling by-products produced during industrial production processes of this type.
The industrial production of sulphur hexafluoride is carried out by means of a reaction of elementary fluorine with molten sulphur. The elementary fluorine is produced by electrolysis of hydrogen fluoride, and always contains -- dependent upon the process of production -- impurities of hydrogen fluoride, oxygen, nitrogen as well as perfluorinated alkanes and cycloalkanes. Furthermore, depending upon trace impurities of the hydrogen fluoride, the fluorine may also contain, for example, the compounds OF.sub.2 (from H.sub.2 O) and ClO.sub.3 F (from chlorine compounds).
In addition to lower sulphur fluorides such as S.sub.2 F.sub.2, SF.sub.4 and S.sub.2 F.sub.10, the sulphur hexafluoride produced by the reaction of fluorine with molten sulphur can also contain a number of further impurities, for example, SF.sub.5 OSF.sub.5 ; SF.sub.5 OCF.sub.3 ; SOF.sub.2 ; SOF.sub.4, SO.sub.2 F.sub.2 ; C.sub.n F.sub.2n.sub.+2 ; C.sub.n F.sub.2n ; ClO.sub.3 F as well as oxygen O.sub.2 and nitrogen N.sub.2. The raw sulphur hexafluoride SF.sub.6 so produced must therefore be purified, in order to fulfill the stringent purity requirements which apply to the use of SF.sub.6 as an insulating and quenching gas.
Normally, the highly toxic S.sub.2 F.sub.10 is removed from the product by means of a down-stream temperature treatment at a temperature of approximately 400.degree. C., in accordance with the equation: EQU S.sub.2 F.sub.10 .fwdarw. SF.sub.4 + SF.sub.6
similarly, hydrolyzable compounds such as SF.sub.4, S.sub.2 F.sub.2, SOF.sub.2, SOF.sub.4 and SO.sub.2 F.sub.2 are removed by means of a subsequently carried out alkali wash and/or alkali absorption step.
Finally, the lower-boiling components such as O.sub.2, N.sub.2 and CF.sub.4 are removed, for example, by being stripped in a first stage of a two-stage pressure distillation step, and the higher-boiling components are concentrated in the column sump of the second distillation stage.
This column sump represents a mixture of different materials and contains, in addition to the principal component SF.sub.6, the higher-boiling impurities, especially SF.sub.5 OSF.sub.5, CF.sub.3 OSF.sub.5, ClO.sub.3 F, perfluorinated alkanes and cycloalkanes. Several of these materials are highly toxic. There exists also the possibility that, in the case of any failure or trouble, e.g., with loss of the temperature treatment or as a result of incomplete washing, other materials may also pass into the column sump, which normally are removed in the upstream purification stages.
The column sump composition is thus a natural waste product, the emptying and storage of which is fraught with risks. As a result of the heterogenous composition of the waste product, chemical destruction to safe products is problematic, difficult and expensive.